This invention relates to a power conversion device and a semiconductor module suited for reduction of power loss caused by noise, size reduction, high-frequency switching, lower noise, and improved reliability of a three-level inverter device.
In a drive control unit for a motor for vehicles, an inverter device is used which can change the frequency of the power supply in order to control the number of revolutions of an a.c. motor. It has been hitherto common that a minimum unit of the inverter device is composed of two switching devices connected in series, which are turned on and off alternately. Instead of the two-level inverter device, attention has recently been focused on a three-level inverter device (also called a neutral-point clamping voltage type inverter), a minimum unit of which comprises four switching devices connected in series (27th Railway Cybernetics Application Symposium, 1990, pp. 198-202). One reason for this is that an output voltage waveform of the three-level inverter device has an intermediate potential point out of a high potential point P and a low potential point N to thereby have a smoothly stepping three-level operation. This has advantages in that low-order harmonics, torque pulsation and noise level can be reduced, in comparison with the conventional two-level inverter devices only having a high potential point P and a low potential point N.
However, compared with the two-level inverter device, the three-level inverter device additionally requires twice as many switching devices and clamping diodes equal to the number of arms of the inverter device. Thus, the three-level inverter device becomes larger in size. Also this increases heat loss in the devices, which causes a problem. The increase of the heat loss requires a large-size cooling apparatus, and, as a result, a system in which the inverter device is used becomes large, which causes another problem. These problems have obstructed the widespread use of the three-level inverter devices.
In addition to the above problems, in case of the three-level inverter device, the use of many devices means that the length of wiring is longer, the wiring inductance is greater, and voltage noise is large, the voltage noise being generated by the wiring inductance and a current change di/dt at the time of switching the switching devices, so that increasing the dielectric strength of the switching devices has found to be necessary by the present inventors. However, increasing the dielectric strength of the switching devices decreases the output current density of the devices, with the result that devices having a larger chip area are required. Therefore, efforts toward the size reduction of the inverter device cannot be achieved.